1. Field of the invention
The present invention relates to a gradient index optical element for use as an optical lens for as a camera, a microscope or the like.
2. Description of the Related Art
A gradient index optical element has a power (refractive power) in its medium by imparting a distribution of refractive index to the medium. This power is determined by the refractive index distribution, so that in order to make the power large, the absolute value .vertline..DELTA.n.vertline. of a difference .DELTA.n in the refractive index n must be rode large. Hitherto, many attempts of making the absolute value .vertline..DELTA.n.vertline. large have been done by many searchers. For example, an optical element which is commercially available under the of SELFOC lens (Tradename) has a large absolute value .vertline..DELTA.n.vertline. made by imparting a concentration gradient of Tl to the medium by ion exchange (Journal of The American Ceramic Society, vol. 54, No. 7(1971), pp321-326). Also a lens of .vertline..DELTA.n.vertline..apprxeq.0.059 was obtained by imparting a concentration gradient of Ag to the medium by a double ion exchange method (Applied Optics, vol. 27, No. 3(1988), pp496-499). Moreover, a lens of .vertline..DELTA.n.vertline..apprxeq.0.04 was obtained by imparting a concentration gradient of Pb and K to the medium by the sol-gel method (J. Non-cry.sol.100, 506, 1988), as well a lens of .vertline..DELTA.n.vertline..DELTA..apprxeq.0.03 was obtained by imparting a concentration gradient of Ti or Ge to the medium by the solgel method (Elect. Lett.22, 99(1986), Elect .Lett.22, 1108(1986)). However, the developments of gradient index optical element up to the present have been mainly an approach of increasing the absolute value .vertline..DELTA.n.vertline., but methods of decreasing the chromatic aberration possessed by the optical elements themselves have not been adequately developed. In designing optical elements, it is possible to drastically decrease the number of lenses used for constructing an optical system for a lens system of a camera using the excellent aberration correction ability of the gradient index optical element, but there is an inconsistency in that the chromatic aberration correction of the lens system becomes more difficult as the number of the lenses decreases.
In order to make a lens systems in which the number of lenses is small and in which the chromatic aberration is corrected, it is important to decrease the chromatic aberration generated in each lens. Therefore, as the gradient refractive index optical element, it is desirable to obtain an optical element having a gradient of a direction A on the n.sub.d -.nu..sub.d graph as shown in FIG. 5, i.e. an optical element having an increasing Abbe number .nu.d as the refractive index n.sub.d increases (refer to Japanese Patent Laid-open No. 141,302/91).
Then, the gradient index optical element is obtained by imparting a gradient of refractive index to the medium on the basis of a gradient of concentration of metal oxide in a glass. The optical property of the glass is determined by its oxide composition. In the case of a glass having its principle component of SiO.sub.2, metal oxides except for Si a have property in that the refractive index is high as compared with SiO.sub.2 glass and dispersion becomes large (that is, the Abbe number becomes small), so that if the gradient index optical element is obtained by imparting a concentration gradient to these metal dopants, only the gradient index optical element having a gradient of a direction B on the n.sub.d -.nu..sub.d graph shown in FIG. 5 is obtained.
Japanese Patent Laid-open No. 141,302/91 discloses a gradient index optical element obtained by imparting a concentration distributions in opposite directions to each other to the medium with the use of two or more metals, in which the chromatic property may be variously changed. However, it is difficult to obtain the gradient index optical element, since the above method of imparting concentration distributions of two metals in opposite directions to each other is limited. That is, hitherto, it is difficult to obtain the optical element having a chromatic property of the direction A on the n.sub.d -.nu..sub.d graph shown in FIG. 5, which is desired in designing of an optical device.
Furthermore, the chromatic property of the gradient index optical element can be variously changed when one of two or more metal dopants is flatly distributed as an absolute amount and the other remaining metal dopant is distributed at a certain slope. A method for attaining such a distribution state may easily be realized as compared with the method of distributing two metals in directions opposite to each other. However, it is still difficult to flatly distribute one metal dopant as its absolute amount.